Heat Damage and Heat Tolerance in Roses

by Jason Capote, Consulting Rosarian, Long Island Rose Society

This is a 2020 AOM winner

Often times in our area we hear of fellow rosarians talk about a rose’s cold hardiness, but rarely do we hear much about its opposite, heat tolerance. Whereas cold hardiness is the degree to which a plant’s tissues or cells can withstand freezing temperatures without being damaged or receiving dieback, heat tolerance is the degree in which a plant can withstand high temperatures, in particular temperatures above 90 degrees for a protracted period of time without dieback or going dormant. In New York City, where I live, considering a rose’s cold hardiness is important (especially in light of the freeze-thaw cycles that are experienced along the Atlantic seaboard from my area to most points due south through northern Virginia), heat tolerance is equally important. NYC frequently experiences temperatures above 90 from late spring through early autumn, and occasionally experiences temperatures above 100 degrees for a protected period of time as well. Additionally, the roses don’t get as much of a break from the heat after sundown, as the high temperatures tend to linger past night fall due to the Urban Heat Island Effect where trapped heat radiates from man made surfaces at night (especially from asphalt streets and the black roofs of houses and apartment buildings) which slows down or even prevents a true cool down. Worse yet, many plants grown in containers in front of buildings along city streets and even along patios in beds up against patio pavers are baked twice from the heat, both from the sun beating down on them and from trapped heat radiating from below from these surfaces. This is not just true for NYC, but is of importance for just about any urbanized metropolitan area that are in regions roughly matching USDA Zones 6 and higher.

Right about now, some of you may be asking why is this all important to know, especially if I live in an area that is not as hot or not as urbanized as some other areas of the country? The answer is simple: with the advent of climate change, average temperatures are slowly rising and as such, an area that is cool today may not be so in the future, especially in regards to summer temperatures as more regions of our own country are experiencing more days above 90 degrees on average for each year for the past decade. As a result it is important to understand what heat tolerance and heat damage are and how we can mitigate the worst effects of the summer heat on our roses.

How Heat Damages Plants in General

The manner in which heat damages plants is two fold. First and foremost, and possibly the most obliviously, heat simply dries out plants and plants receive damage from desiccation and will also “wilt” from a drop in their internal turgor pressure (the pressure exerted by water on the cell walls of plants which keeps them standing upright with plump leaves and flowers). Secondly, heat damages plants by slowing down the enzymatic chemical reactions that occur in plants that sustain their life processes. In some extreme cases, excessive heat can even denature the enzymes completely resulting in a total stoppage of life functions resulting in serious dieback of plants and even death.

While the first problem of desiccation is a concern, it is readily fixable by just watering plants more heavily in hot weather and in most cases, any damage done from this is easily reversible if adequate water is supplied in a timely fashion. The second problem of enzymatic reactions slowing down or even stopping in the event of denaturing is not easily reversible. When the enzymes slow down, the plant goes into a period of what horticulturalists call “heat dormancy” where plants simply stop growing, but are otherwise still alive. This too is reversible, but the plant has to wake up from this on its own as there is little a gardener can do in this situation. Eventually the temperatures will fall, the plant cools down, and the enzymes resume their normal functions and the plants begin to grow again. Damage is only sustained in this case if the period of heat dormancy is protracted enough that the plant runs out of stored food to live off of, but thankfully for roses, they store ample amounts of food in healthy canes to usually withstand this. The more serious case of enzymes breaking down and chemically altering in the denaturing process is what we call in layman’s terms say is “cooking.” That is right, the entire cooking process is predominantly the result of heat denaturing enzymes and other proteins to chemically alter them, and it is not reversible. Just like you can’t “uncook” food, you cannot undo the damage caused by denaturing enzymes which is why plants that experience this phenomena are usually down for the count when this process happens.

Heat Tolerance and Heat Damage in Roses in Particular

In terms of roses in particular, as implied earlier, roses have certain defense mechanisms that they can naturally employ against heat damage as a result of evolutionary advantages similar to how they have defense mechanisms to protect them from a protracted freeze. First and foremost, as mentioned earlier, roses store food and water in their canes. This storehouse of water helps them to maintain internal turgor pressure in the face of damage from desiccation. As a result, heat damage, especially during drier periods, is usually first noticed in the wilting of flower petals as opposed to the leaves of the bush making the petals look almost like paper and also resulting in blooms either opening too fast (what some rosariars dub as “blowing open”) or simply dropping petals in the heat. One way expert rosarians handle this problem in areas that are even hotter than our region, especially in more arid climates, such as the American Southwest, is to select roses that have flower petals that have high substance. When petals have a high substance, they have high amounts of starch, which helps them hold onto water better. It also gives them a much more rigid appearance overall which allows them to hold their form better and as a result, many of the top exhibition roses are ones with a high degree of petal substance.

In terms of heat-induced dormancy as a result of enzymes slowing down their chemical reactions, roses will simply stop actively growing. Like all plants, the blooms of a rose are ultimately their reproductive organs, and like all life forms, while reproduction is important to secure a species’ future for another generation, sustaining one’s own life is more important as a life form cannot reproduce if it is dead. As a result, when roses begin their process of entering heat dormancy, they will first stop blooming. Then, if the heat continues, they will stop actively growing: they will not grow from terminals; they will not send out basal breaks; they will not replace dead or damaged leaves. It is as some gardeners dub “the rose is just sitting there.” In cases of protracted periods of excessive heat (such as long heat waves of days over 100 degrees) or on long periods of heat coupled with drought or drought-like conditions, the roses will even drop some or even all their leaves, which one rosarian I know affectionately dubs “the rose dropped its shorts.” While some rosarians may be alarmed at seeing this, this is nothing to seriously worry about as, once again, this process is another defense mechanism that roses have evolved to help them survive this weather condition. They do this primarily to hold onto water that they lose through transpiration (which is where plants lose water from their leaves as water vapor as a result of photosynthesis). Once again, because roses store food and water in their canes and roots, they can survive this extreme version of heat dormancy for quite some time without serious damage. In fact, most modern roses can even replenish a small amount of this stored food by photosynthesizing using the chlorophyll in their canes (hence why most modern roses have green canes).

Conclusion – What You Can Do to Help Out Heat Stressed Roses

By and large, while a rosarian cannot on their own wake up a rose from heat dormancy, and while they cannot truly even prevent it, there are a few methods that they can do to help their roses through this rough period and can also serve to mitigate the worst effects of it. First and foremost, on days above 90 degrees, unless a torrential downpour came over your area, you need to water your roses. Having ample supplies of water will help keep the roses from dropping their leaves and will keep those that are still blooming from dropping their flowers or aborting their flower buds from desiccation. Second, when watering your roses, you can apply the water overhead once the shade passes over them. This will help them cool down which will also prevent any heat induced damage and which may also prevent them from dropping their leaves when they go heat dormant. Normally we tell rose growers not to water overhead as a general rule but here is an exception. Don’t worry so much about fungal diseases as when it is over 90 degrees, most of the water will evaporate off the leaves so fungal spores will not spread. In fact, it is this process of evaporation that cools down the bushes similar to how evaporative cooling cools you down when you sweat. Lastly, you can plant roses that are much more heat tolerant. One key indicator to whether a rose is heat tolerant is to look for bushes that regularly produce thick, leathery foliage as this type of leaf is less likely to lose water from transpiration. One important thing to keep in mind however is that many roses that are heat tolerant are not very cold hardy and vice-versa, so a rosarian needs to do their research.

Lastly, in terms of damage caused by periods of high heat denaturing the enzymes of plants thereby killing them, this is quite rare in roses as most varieties can withstand this even under the most pressing of heat conditions, but it is not impossible. A few varieties literally cook in the heat. I unfortunately had this experience with a beautiful David Austin rose called ‘Fighting Temarie’ which is a highly fragrant, single petaled rose that had its canes literally go limp and blacken in the sun during a 100 degree day. Each day it would continue to seriously dieback no matter how much water it was given despite the fact that it was planted in a location that has afternoon shade. The only solution to this problem is to simply not plant roses that experience this issue, especially in hot climates.

ABOVE: In order of appearance, Gemini, Chihuly and Veteran’s Honor, by Sue Streeper